I am drawing graphics (bars) with GD::Graph::bars. But I can't fill bars with gradient, only solid color. Saw to the php side but I want explore the modules. I want fill bars with gradient.

May be someone know about it: GD::Graph::bars drawing polygon with GD. GD has method fill which fills limited area by point inside area. Where I can found the code of filling process? For correct it: write mathematic function of filling that in bars has gradient?